1. Field of the Disclosure
The present invention is generally directed to a process for making anodized aluminum articles, and more particularly to an anodizing process for producing articles having a heat stable, colored, hard anodized surface, to a method of making heat stable colored cookware, and to such articles.
2. Description of Related Art
Aluminum metal is readily oxidized. A natural oxidized film occurs on the surfaces of aluminum. The naturally occurring oxide is corrosion resistant, very hard, abrasion resistant, durable, and acts as an insulator to the aluminum. However, the natural occurring form of an oxide film is quite thin. Thus, the natural oxide film can be damaged easily. Pure aluminum naturally produces a highly corrosion resistant aluminum oxide layer, but the layer is very thin. Aluminum alloys naturally produce a slightly thicker aluminum oxide layer, but the layer is less corrosion resistant. Anodizing is a well known process used to enhance the thickness and other characteristics of this oxide film on the surfaces of an aluminum substrate. Aluminum alloys are thus anodized to enhance the corrosion resistance of the oxidation layer, to increase the hardness of the oxidized layer, to allow coloring or dying of the layer, to improve adhesion of the oxidized layer, or the like.
Different anodizing processes are also known. Chromic acid or Type I anodizing processes are old and well known and produce a thin oxide layer on an aluminum substrate that is difficult to dye or color. The process also requires a relatively high-voltage. Sulfuric acid anodizing (Types II and III) is also well known. Sulfuric acid solutions are commonly used for Types II and III anodizing processes, but other solutions are also known and used. Type II anodizing produces a thinner coating on the order of about 1.8 to about 25 μm and is typically conducted in a tank with an electrolytic solution in the range of about 70° F. Type III anodizing produces a thicker coating on the order of about 25 μm and up. Type III anodizing is known as hard anodizing, which is typically conducted in a tank with an electrolytic solution near 32° F. and requires higher voltages than the thinner coatings of a Type II anodizing process. Hard anodizing increases the wear resistance and corrosion resistance of the anodized layer and thus produces a more durable oxidized layer on the aluminum substrate. However, hard anodizing also produces a dark gray colored anodized surface.
Color anodizing is very common in architectural or Type II anodizing processes. Color anodizing is not as common for Type III anodizing or hard anodizing of aluminum. It can be very difficult to color hard anodized aluminum articles because the process produces the very dark gray surface color. The hard anodized surface does not take on color or dye easily.
Manufacturers that have created aluminum cookware know well that, if the anodized layer is colored, the color will fade almost immediately when subjected to relatively high temperatures unless protected in some manner. Thus, anodized aluminum cookware is typically colored by painting the aluminum article after the anodizing process. Painting the cookware requires subsequent manufacturing steps. Some cookware is color anodized, but then an oxygen barrier layer is typically added to the article after the anodizing process to protect the color. This again requires additional manufacturing steps subsequent to anodizing. These paint or barrier layers have been found to wear quickly and/or become easily damaged during normal use.
Many types of cookware have a copper coloring that is highly desirable to consumers. Some cookware of this type is painted after being anodized to add the copper coloring. Some cookware may have an anodized copper coloring or copper surface, which then is protected by a subsequent coating or oxygen barrier layer. The protective layer is applied to the exterior surface of the cookware, after the anodizing process, to reduce or inhibit fading of the copper color during normal use.
As noted above, it is well known to color anodize aluminum of the type II or architectural variety. U.S. Pat. No. 5,587,063 discloses a process for electrolytically coloring anodized aluminum surfaces during an architectural or type II anodizing process. The '063 patent discloses the combination of a copper layer and a tin layer to produce a copper/bronze color. However, a copper/bronze colored aluminum article produced by such a Type II anodizing process would not have a very durable anodized layer. Also, the anodized, colored surface would not be suitable to application of sustained heat over 375° F., as the color would rapidly fade.